1. Field of the Invention
Embodiments of the present invention generally relate to filters assemblies, such as those used to filter lubricants. Particular embodiments of the present invention relate to plastic filter assemblies with replaceable plastic filter elements. The invention is broadly applicable and can be used in hydraulic, fuel and air filter applications.
2. Discussion of the Related Art
In order to remove contaminants from a flowing gas or liquid, the contaminated medium is often passed through a filter element in a filter. Filters are commonly used in the lubrication systems of standard internal combustion engines, e.g., automotive engines, truck or heavy equipment engines, and stationary power sources i.e. computer numerical control CNC machines, injection molding, die cast machines, compressors, etc.
Filtration systems used in these applications generally include a cylindrical housing into which a cylindrical filter is placed to remove particulate materials from fluids such as water or air. Two types of filter assemblies have commonly been used in lubrication system applications, filter assemblies with removable filter elements and disposable filter assemblies. In a commonly used “spin-on” disposable filter assembly, the filter element is sealed in a metal can with a metal core located in the center of the element for support structure. In such systems, to replace a clogged or dirty filter element, it is necessary to replace and dispose of the entire filter assembly.
In many filtration applications, the filter element must be changed periodically. For instance, in automotive applications the oil filter is typically changed every few thousand miles or every few months. There are a limited number of reusable oil filter types available or in use, but in most high quality lubrication systems, spin-on disposable filter assemblies are used, and these can create a disposal problem and are treated as hazardous material.
When filters were first introduced for use in lubrication systems, it was common to utilize cartridge type filter elements that fit into a removable housing. When the filter element needed replacement, the housing was removed from the oil filter mount on the engine, the cartridge was removed from the housing, the housing was cleaned, a new cartridge was installed, and the housing with the new cartridge was then replaced on the engine. Cartridge filters of that type usually included a cellulose filter membrane, exterior metal support, and a supporting center tube, typically of metal mesh or expanded metal. The metal supports, the center tube or outer wrap, were needed to prevent the filter from being crushed by the pressure generated in the lubricant being filtered. Differential pressures in an automotive hydraulic system can rise substantially at engine start-up, and particularly during malfunctions, such as a plugged filter malfunction (due, for example, to water or excess engine wear metals in the oil), and can reach 200 pounds per square inch (psi) or more.
Conventional practice in the past required the use of a support tube in combination with cellulose/glass fiber filters. The filter elements provided good filtering capability, and the metallic supporting structure provided the necessary rigidity and resistance to buckling due to the differential pressure between the inlet and outlet sides of the filter membrane. Disposal of the cartridge was complicated by the rigidly attached metal supporting structure that made crushing impractical and complete incineration impossible.
In more modem lubrication systems, spin-on disposable filter assemblies have been used. Spin-on disposable filter assemblies are typically more expensive, and create a greater disposal problem. However, the simplicity of removing an old filter and spinning a new one on in its place has overcome these drawbacks in many commercial applications. The spin-on filters include the typical cellulose filter elements, as well as an external shell of sheet metal, a center supporting tube, a threaded base plate and any necessary structure to hold the filter in place and prevent its damage. After it is used, the entire spin-on filter, metal shell and all, must be discarded.
Environmental regulations, the limited availability of landfills, and a greater awareness on the part of the public with respect to landfill pollution have created the need for an oil filter of the type which can be safely disposed of in an environmentally acceptable way. The canister type spin-on disposable filter assemblies are problematic because they have a substantial metal content, along with the paper content, gasket content, and residual oil. Even the older variety of cartridge type filters has disposal problems, because they contain both metallic parts (for support) as well as the cellulose parts (for filtering).
Attempts have been made to produce an environmentally disposably acceptable filter, (i.e., an environmentally friendly filter) but they have also suffered their drawbacks. For example, it has been proposed to utilize a filter cartridge with no metallic center support tube, and build the support tube into the filter housing. However, these approaches have been less than satisfactory for a number of reasons.
One type of spin-on filter with a replaceable/disposable filter cartridge designed to address these problems uses a radial seal as the main seal between the interior and the exterior of the filter element. However, a problem encountered when using a radial seal as the main seal involves the difficulty of disassembling the filter housing in order to change the cartridge. This type of sealing arrangement requires an unusual amount of torque to detach the cover from the housing. Even more significantly, while the center support tubes provide protection from crushing the filter paper elements in the radial direction, the filter element experiences significant pressure drops along its axis. Those pressure drops can be large enough to either unseat the filter and cause leakage around the main seal at one or the other end cap, or to begin to compress or crush the filter along its axis. Thus, although these filter cartridges have no metallic parts to complicate disposal, the filters themselves have significantly inferior structural properties and shorter lifespans as a result.
It is possible, by making certain compromises, to compensate for the lack of strength of an unsupported filter cartridge by using bypass valves either in the filter or in the engine. The function of a bypass valve is to respond to a pressure differential buildup caused, for example, by a plugged filter, and bypass oil around the filter. In effect, the bypass valve limits pressures in the system, but at the cost of passing unfiltered oil to the equipment. That might be acceptable in an automotive application, but in other applications it is completely undesirable. For example, a pressure relief valve is undesirable in those cases where passing unfiltered fluid might do permanent damage to the machinery being protected. Typical examples are a diesel fuel system or a hydraulic system. In such systems, it is considered preferable to allow the filter to plug to protect the equipment from a catastrophic and costly failure. To withstand the pressures as the filter plugs in such systems, the filter cartridge must have adequate structural support, which eliminates the possibility of using the unsupported filter cartridges that have been available in the past.
U.S. Pat. No. 5,556,542 (“the Berman et al. reference”) discloses a snap-together, all-plastic filter assembly for filtering fluids that includes a cylindrical injection-molded plastic outer shell with a closed base and an open opposite end and which defines a hollow interior which receives a filtering element and an integral injection molded plastic endplate/center tube member. The outer shell is injection molded with a pair of concentric, generally cylindrical, inner annular walls which are integral with the closed base and extend part way toward the open end of the outer shell. The filtering element which has a hollow interior fits down within the outermost of the two concentric annular walls and the center tube of the endplate/center tube member extends through the center of the filtering element and snaps in place by means of snap-fit projections which snap into snap-fit pockets disposed within the inner most of the two concentric annular walls. The filter assembly is designed as a spin-on filter and is threadedly engaged and positioned onto a mounting base thereby completing the fluid flow path.
However, a major concern with plastic filter assemblies is the propensity of the filters to “grenade”, i.e., explode into fragments that may damage the filter element or surrounding equipment. Therefore, there is a need for a safe, environmentally-friendly lightweight filter assembly that requires replacement and disposal of only the filter element, and that is not subject to grenading.